1. Field of the Invention
The present invention relates to apparatuses and methods for controlling actuation of valves of internal combustion engines in general, and, more particularly, to a variable valve actuation system adapted to provide various operating modes of an internal combustion engine including compression release engine braking.
2. Description of the Prior Art
Most commercially available automotive engines operate with fixed valve lift profiles to provide for fresh air intake and exhaust gas discharge. This fixed lift, duration and timing of the valve events results in compromise among the competing performance factors of engine power density, fuel economy and exhaust emissions. Many benefits can be realized if the valve events are made variable and optimized for particular operating modes of the engine.
The two-mode system of Bhargava et al. (U.S. Pat. No. 6,092,496) opens the intake valve during the exhaust stroke during warming-up of the engine. This directs a portion of the hot exhaust gas to the intake manifold, which mixes with the incoming fresh air and provides a warmer charge to the cylinder during the main intake stroke. This mode is invoked whenever a sensed engine associated temperature falls below a predetermined threshold level.
The valve control apparatus of Meneely et al. (U.S. Pat. No. 6,314,926) operates by means of dynamic lash adjustment to engage with one or two lobes on a cam profile. One lobe is to actuate the main intake or exhaust event. For the exhaust, the second lobe may be a compression release lift profile for engine braking. When the engine brake mode is on, the main exhaust opening is also advanced. Provision is specifically made to disengage the lash adjustment before the main exhaust achieves full lift, thereby returning the system to a normal exhaust valve opening and a normal valve overlap with the intake valve opening. Since the main exhaust valve opening (EVO) is advanced only when in engine braking mode, advantage cannot be taken of the early EVO during positive power to enhance turbocharger turbine response.
Usko (U.S. Pat. No. 6,354,254) has developed rocker assemblies to modify valve lift and timing. Two main rockers are used for positive power modes. Full exhaust valve lift (EVL) includes an opening during the intake stroke for internal exhaust gas recirculation (EGR). Reduced EVL eliminates the EGR opening. Full intake valve lift (IVL) increases valve overlap and reduced valve lift gives an early valve closing. In this system, the lash adjustment means to change operating mode for the engine is limited to two positions. The EGR provided for positive power is not compatible with engine braking, so a braking lobe cannot be included on the exhaust cam profile. A third rocker is required to provide engine braking, with a cam dedicated for this process. It includes a compression release lobe and another lobe for exhaust gas recirculation during braking, called brake gas recirculation (BGR). This extra mechanism and cam takes up valuable space in the engine and is a significant added cost.
Many approaches have been taken to develop variable valve actuation with infinite adjustment means. These systems necessarily use electronic controls to optimize the intake and exhaust valve lift profiles, based on demand from the engine. These control systems represent added complexity and cost in return for some extra fine-tuning of specific engine processes. Simko (U.S. Pat. No. 5,161,497) describes a method for phase shifting the exhaust and intake events to reduce pumping losses and improve exhaust emissions. Mikame (U.S. Pat. No. 6,244,230) developed a workable phase shifting system with dual camshafts. Another mechanical variable valve actuation (VVA) system, by Nakamura (U.S. Pat. No. 6,390,041), does not shift the phase of the valve openings, but has the ability to change the valve opening magnitude from full lift to zero lift. Opening and closing points for exhaust and intake events can be varied, centered on constant crank angle timing of the peak lifts.
For internal combustion engines, especially diesel engines, engine braking is an important feature for enhanced vehicle safety. Compression release engine brakes open the exhaust valve(s) prior to Top Dead Center (TDC) of the compression stroke. This creates a blow-down of the compressed cylinder gas and the energy used for compression is not reclaimed. The result is engine braking, or retarding, power. A conventional engine brake has substantial cost associated with the hardware required to open the exhaust valve(s) against the extremely high load of a compressed cylinder charge. The valve train components must be designed and manufactured to operate reliably at high mechanical loading. Also, the sudden release of the highly compressed gas comes with a high level of noise. In some areas, engine brake use is not permitted because of the loud noise, establishing a potential safety hazard.
Exhaust brakes can be used on engines where compression release loading is too great for the valve train. The exhaust brake mechanism consists of a restrictor element mounted in the exhaust system. When this restrictor is closed, backpressure resists the exit of gases during the exhaust cycle and provides a braking function. This system provides less braking power than a compression release engine brake, but also at less cost. As with a compression release brake, the retarding power of an exhaust brake falls off sharply as engine speed decreases. This happens because the restriction is optimized to generate maximum allowable backpressure at rated engine speed. The restriction is simply insufficient to be effective at the lower engine speeds.
While known valve actuation systems, including but not limited to those discussed above, have proven to be acceptable for various vehicular driveline applications, such devices are nevertheless susceptible to improvements that may enhance their performance and cost. With this in mind, a need exists to develop improved variable valve actuation systems and driveline apparatuses that advance the art, such as a modal variable valve actuation system that can provide two or more modes of operation for the exhaust valves and for the intake valves, in order to optimize a range of processes in an internal combustion engine. A practical system will use step-wise switching and will not incur the high cost and reliability issues of high-speed actuators and their associated electronic controls. Engine braking must be provided as an integral feature for internal combustion (I.C.) engines and not require additional valve actuation apparatus. The engine brake will incorporate a quiet process to be useful in environments sensitive to noise pollution and will operate with reduced mechanical loading on the engine. The valve lift modes for powering the engine will provide the benefits of enhanced power density and fuel economy and improved exhaust emissions for targeted ranges of engine operation.